In the past year, the Cytotoxic Cell Studies group has made a major discovery that explains the phenomenon of selective gene activation. We have discovered a probabilistic transcriptional switch that controls Ly49 gene activation, and it appears that the separately evolved human KIR gene family uses the same type of switch, indicating that probabilistic switches will likely be involved in many systems where genes are selectively activated in a subset of the cells in a given tissue. This discovery has important implications for the control of stem cell differentiation, and may one day allow us to modify cell fate in differentiating systems such as bone marrow cultures.